Preparation of wood pulp



PREPARATION OF wool) PULP Christopher F. Kearton, John H. Givens, and Fred C.

Aldred, all of Coventry, England, assignors to Courtaulds Limited, a British company, London, England Filed Aug. 27, 1957, Ser. No. 680,459 Claims priority, application Great Britain Oct. 2, 1956 3 Claims. or. 34-10 This invention relates to the preparation of wood ulp. p In the viscose industry the usual form of cellulosic starting material is wood pulp in the form of sheets. In the production of such sheets, wood is chemically converted into pulp e.g. by the sulphite, sulphate or soda process. After removing the excess pulping agent the aqueous wood pulp slurry is usually converted into sheet form by draining suflicient Water from the slurry on a wire mesh belt to form a moist self-supporting sheet and then completing the drying by passing the sheet through a series of heated rollers. In the manufacture of viscose these sheets are shredded either before or after the conversion of the cellulose into alkali cellulose.

The object of this invention is to produce wood pulp in a discrete form suitable for use in the manufacture of viscose.

According to this invention wood pulp'is obtained by forming a wood pulp slurry by standard methods, reducing the moisture content of the slurry to within the range of 15 to 75 percent by weight based on the weight of the dry pulp and completing the drying by fluidizing the pulp with hot air supplied under a fluctuating pressure suflicient to cause the fluidized pulp particles to vibrate and thus prevent coalescence of the particles.

When the pulp slurry has been prepared the removal of water to the range of 15 to 75 percent as defined may be eflected in a slurry press, a screw press, or a continuous centrifuge. After the excess water has been removed in this way it is preferred to add hot water to the pulp to form a fresh slurry which is then de-watered to the 15 to 75 percent range in the same way.

The drying of the pulp in the fluidized state may be elfected using hot air with an air velocity of 1 to 2 ft./sec. and causing the particles to vibrate by varying the pressure on the air supply by a frequency of a few cycles per second.

The invention is illustrated by the accompanying flow sheet in the description of which the percentages are all calculated by weight on the weight of the dry pulp. A 1 percent slurry enters the process through a rotary filter where the moisture content is first reduced and is almost immediately raised by adding hot water to produce a slurry containing about 90 to 95 percent of water. Thence the slurry passes through a pump 1 and a storage tank to a press where the water content is reduced to between 15 to 75 percent, preferably 45 to 50 percent. The water pressed off is returned to the filter after passing through stages described below. The moist pulp from the press now passes to a shredder which breaks up the moist pulp. The moist, shredded pulp passes by way of a cyclone A to the dryer. in the dryer, pulp is dried in the form of a fluidized bed and a pulsating, hot air supply is used to fluidize the pulp in order to prevent coalescence of the particles during drying. The dried pulp passes to a baler. Subsidiary circuits are followed by the air containing some pulp dust and the water.

States Patent 2,974,420 Patented Mar. 14, 1961 The hot air from the dryer passes to a cyclone B. Here some of the entrained pulp dust is removed and. passes to the baler. The air from cyclone B passes through a heat exchanger with the incoming air for the fluidized bed. The outgoing gas is then scrubbed with water coming from the press mentioned above and finally joins the stream of incoming air. The incoming air passes via a fan to the heat exchanger and then on to a heater and the dryer. In the water circuit the water at the first filter passes with the slurry through a pump 1, tank and press as described. It then passes via a sump and a pump 2 to be used in the scrubbers for theoutgoing air, and returns to the filter via a storage tank.

The invention is illustrated in the following example.

Example Wood pulp having a water content of 47.5 percent by weight on the weight of the dry pulp was fed to the dryer in which a pulsating air supply at a temperature of 90 C. and at a mean air velocity of 1.2 feet per second fluidized the pulp to form a bed 4 inches deep. The coalescence of pulp particles was prevented by the pulse in the air supply which had a frequency of 4 cycles per second. After 2 minutes treatment the pulp had a water content of 11.7 percent and after a further 2 minutes treatment the water content of the pulp had fallen to 0.06 percent. 1

What we claim is:

1. A process for drying particles of wood pulp containing from about 15 to about water on the weight of dry pulp which comprises feeding said pulp to a fluidized bed of pulp and supplying hot air to said bed at a. velocity of between about 1 and about 2 ft./sec. and at a pressure which fluctuates at a frequency on the order of 4 cycles/sec. and sufliciently to cause the particles of pulp to vibrate, thus preventing coalescence of said particles.

2. A process for drying an aqueous wood pulp slurry which comprises mechanically reducing the water content of said slurry to between about 15 and about 75% by weight of the dry pulp, feeding the pulp to a fluidized bed of pulp and supplying hot air to said bed at a velocity of between about 1 and about 2 ft./sec. and at a pressure which fluctuates at a frequency on the order of 4 cycles/sec. and sufliciently to cause the particles of pulp to vibrate, thus preventing coalescence of said particles.

3. A process for drying a wood pulp slurry which comprises mechanically reducing the water content of the slurry, adding hot water to the resulting pulp to reform a slurry, mechanically reducing the water content of the reformed slurry to between about 15 and about 75 by weight of the dry pulp, feeding the resulting pulp containing 15 to 75 water to a fluidized bed of pulp and supplying hot air to said bed at a velocity between about 1 and about 2 ft./sec. and at a pressure which fluctuates at a frequency on the order of 4 cycles/sec. and sufiiciently to cause the particles of pulp to vibrate and thus prevent coalescence of said particles.

References Cited in the file of this patent UNITEDSTATES PATENTS 663,868 Crean Dec. 18, 1900 1,472,314 Webster Oct. 30, 1923 1,774,309 Anderson Aug. 26, 1930 2,102,427 Lloyd et a1 Dec. 14, 1937 2,409,747 Folmer Oct. 22, 1946 2,513,369 Shaw July 4, 1950 2,667,706 Morse et a1 Feb. 2, 1954 2,813,351 Godel Nov. 19, 1957 

